Ceramic honeycomb structures are widely used as anti-pollutant devices in the exhaust systems of automotive vehicles, both as catalytic converter substrates in automobiles, and diesel particulate filters in diesel-powered vehicles. In both applications, the ceramic honeycomb structures are formed from a matrix of thin ceramic webs which define a plurality of parallel, gas conducting channels. In honeycomb structures used as ceramic catalytic substrates, the cell density may be as high as about 900 cells per square inch. To reduce the pressure drop that the exhaust gases create when flowing through the honeycomb structure, the web walls are rendered quite thin, i.e. on the order of 2-6 mils. Ceramic honeycomb structures used as diesel particulate filters have a lower cell density of between about 100 and 400 cells per square inch, formed from webs on the order of 12-25 mils thick. In both cases, the matrix of cells is preferably surrounded by an outer skin which is quite thin, i.e. generally two or three times thicker than the web walls.
Such ceramic honeycomb structures are typically manufactured by way of a procedure in which conveyor units in the form of trams, trays or pallets of a conveyor system continuously move unfinished ceramic bodies through a number of stations, hereinafter referred to as “manufacturing loops”, each of which completes one or more necessary manufacturing steps. In the first steps of the manufacturing process, the ceramic ingredients are pulverized and mixed together with a binder to form a paste-like substance which is extruded into a honeycomb body. The extruded honeycomb body is cut into segments that form green ceramic bodies, which are then loaded into the automatic conveyor system. The conveyor system then slowly moves these green bodies through a kiln, where they are fired at temperatures of typically 1300° C. or higher in order to fuse the batch constituent particles present in the extruded material into a ceramic fired body. The ceramic fired bodies may then be conveyed to a contouring station where all the outer skin of the body is abraded off, and replaced with a new skin that meets precise dimensions. The conveyor system may then move the contoured ceramic bodies through another kiln in which they are fired again at lower temperatures, for example, on the order of 800° C. or more, and from there to a coating station that coats the gas contacting surfaces with a washcoat that may contain catalytic metals. Finally, the automatic conveyor system moves the finished ceramic structures to a packaging station where they are packaged and arranged for shipping.
Due to the thinness of the outer skin and the inner cell-forming webs, the substantial thermal stresses that the unfinished ceramic structures undergo during the firing processes, and the necessary mechanical handling of the green and fired bodies during the manufacturing process, defects such as internal cracks and voids may occur, as well as separations between the outer skin and the inner matrix of webs. To reduce the occurrence of such defects, it would be desirable to have a quality control procedure which allowed the manufacture to reliably trace any defective ceramic honeycomb structure back to the specific factory, kiln, and batch that it originated from. Such a procedure would allow the manufacturer to review the particular manufacturing parameters used to fabricate the defective unit and to modify its manufacturing operation in order to reduce the occurrence of such defects in future articles. Accordingly, it is a known procedure to mark, after the final firing or heating step, finished ceramic honeycomb structures with marks containing manufacturing information so that remedial manufacturing operations may be implemented.
Unfortunately, the applicants have observed that such a marking procedure does not reliably result in an accurate recovery of the manufacturing information associated with a particular ceramic honeycomb structure. In particular, the applicants have observed that subsequent to the manufacture of the green bodies of such structures, different batches of fired bodies from different kilns necessarily become mixed together in order to efficiently implement other stages of the fabrication process. Additionally, different unfinished ceramic structures may be removed from one or more manufacturing loops, put into storage, and then reintroduced either onto the conveyor system or directly into another manufacturing loop. Hence a quality control process where manufacturing information is printed on the finished ceramic honeycomb structures may not accurately reflect the actual manufacturing conditions and history of the structures, as structures which end up adjacent to one another in the final conveyor units might have quite different manufacturing histories.
Some of the aforementioned problems could be avoided by printing a data-carrying mark on the side wall of the green bodies that ultimately form finished completed ceramic honeycomb structures. Such a mark could include a detailed manufacturing history of the structure. However, even the best high-temperature inks known in the prior art fade or run when subjected to the 1100° C. to 1460° C. temperatures necessary to fire the green bodies into the ceramic structures, thereby making it difficult for the mark (whether in the form of a bar code or alpha-numeric code) to maintain the number of characters necessary to accurately preserve a detailed manufacturing history of the ceramic structure. And even if more durable high-temperature inks are ultimately discovered, there are some steps in the manufacture of ceramic honeycomb structures, such as contouring, which require the outer skin of the structure to be removed, thereby unequivocally obliterating any data-carrying mark printed on the outer surface of such structures.
Accordingly, there is a need for a system and method for reliably and accurately preserving both identification and manufacturing information throughout the manufacture of an unfinished honeycomb ceramic structure, and in particular throughout mark-obliterating manufacturing steps such as firing and contouring. Ideally, such a system and method would be easy and inexpensive to incorporate into presently-used ceramic manufacturing facilities, and would not result in production bottlenecks which would slow the rate of production. Finally, it would be desirable if the amount of manufacturing information preserved for each individual ceramic structure was not confined to the informational limits of a bar code or other type of data-carrying mark that could be rapidly and easily printed on the side of a ceramic structure during manufacture.